A rehearsal has two jobs, and they pull in opposite directions. It has to fix the message so hard that you could deliver it with the deck switched off. It also has to leave the sentences loose enough that an interruption doesn't derail you. Memorising words does the first job badly and the second not at all.
What should you memorise, and what should you leave loose?
What you memorise is a short list. The one-sentence takeaway, in the exact words you'll say it. The three claims that carry it. Each claim's number, to the precision you'll speak it. Their order, and the reason for that order. And the ask at the end, again in the exact words. People fumble the ask more often than any other sentence, and it's the sentence the meeting exists for. That's roughly forty words of hard memory for a twenty-minute talk. It's also the only part where recall under pressure has to be perfect.
Everything else you rehearse as behaviour. You know that slide 6 is where the margin bridge lands, and that you say the 380 basis point number out loud and then stop talking. You don't know which of four sentences you'll use to get there, and you don't need to.
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How many times should you rehearse a presentation?
Four passes, and what each one is for.
Pass one is a silent read-through at your desk. It catches missing slides, a chart with no title, a number that appears twice with two different values. It tells you almost nothing about time. On the twenty-two slide deck in the third worked example, the silent pass ran 913 seconds and the stopwatch pass ran 1,517 seconds. So the desk read understated the real runtime by 604 seconds, or 39.8 percent of it. Time a talk by reading it in your head and you'll be a third over on the day without knowing why.
Pass two is standing up, out loud, with no stops. Don't mutter and don't read from anything. Speak at delivery volume, in a room. On the same deck this pass ran 1,442 seconds. Its job is to find the places where you don't actually know what you think. They announce themselves as a sentence that starts three times.
Pass three is the stopwatch pass, one time per slide, notes closed. It ran 1,517 seconds, 75 seconds longer than pass two. You might expect a stop-start run to come in shorter, so why is it longer? Closing the notes costs recall time. And stopping the clock on each slide makes you finish each thought before the next slide comes up. Pass three is the number to trust, and it's the one you plan the cut from.
Pass four happens after the cut, and it's the one most people skip. You open cold with no warm-up, run the deck from slide 1, and have whoever is helping you plant a question in the middle. On the worked deck the post-cut runtime was 1,023 seconds of slides plus a 45 second planted question. That's 1,068 seconds against an 1,080 second slot, so 12 seconds of margin. Uncomfortably tight? It's meant to be, because the interruption is already in the measurement.
All four passes cost 4,940 seconds, which is 82 minutes and 20 seconds, or 4.63 times the length of the talk you eventually give. So a twenty-minute presentation isn't twenty minutes of rehearsal. Budget an hour and a half, and be pleased when it takes less.
What can't rehearsal fix?
There's a limit to all of this. Rehearsal makes the timing reliable and the opening fluent, but it can't make a weak argument strong. If pass two keeps stalling in the same place, look at the logic before the delivery. Usually slide 8 doesn't follow from slide 7, and no amount of practice will make a gap in the logic sound smooth.